Previously, aiming at the problem of the prior electric cutting tool with an irremovable motor, the applicant had provided a combined operation power gun with a changeable motor and obtained the Chinese patent with the patent number being 200520144724.X. In the patent, electric connection of the motor is realized by the means that two ring-shaped electrodes arranged on the end part of the motor are respectively in contact with two needle-shaped electrodes arranged on a housing, The configuration can avoid misconnection for the electrodes without affecting the connection between the electrodes at any angle that the motor turns relative to the housing, however, because the electrode rings on the motor can not be designed wide enough and the needle-shaped electrodes on the housing can not be designed thick enough, the motor must be accurately located in the housing in order to ensure good contact between the electrodes, otherwise the contact area of the two electrodes may be changed thereby affecting electric conduction effect. In the patent, in order to prevent the motor from being freely rotated in the housing, a location lockgroove is arranged on the housing, a corresponding locating lug boss is arranged on the motor, and the motor is circumferentially fixed relative to the housing by matching the lug boss with the lock groove. Because the lock groove is arranged at the bottom of the cavity of the housing, the motor needs to be turned after being installed to align the lug boss to the lock groove, while only a little of the back end of the motor is exposed, therefore, the location mode causes some inconvenience to the assembly of the motor. Further more, in the specific embodiment of the specification, the patent describes a specific structure of a back cover, wherein the back cover is locked in the lock groove arranged on the housing by the lug boss arranged thereon; because the lug boss in the specific structure is a fixed structure, the locking assembly between the lug boss and the lock groove is getting more and more loose after being opened and closed at several times thereby finally affecting the reliable close of the back cover.